Quick but annoying problem

Hello developers!
I am using Xcode 4 and have run into a little bit of a problem. Firstly, I have been compiling and building mac apps easily in Xcode 3. But when I try to compile in Xcode 4, I get this error of a code sign error and the certificate for developing doesn't match. I can't find a way to fix it.

And secondly, some projects don't have this error. So when I try to implement a text view and a toolbar in the xib, it compiles normally but does not run. It runs for like a millisecond and then quits. So please help me with that.

It's now driving me crazy, as I can't get anything done. A shame, because Xcode 4 has an awesome interface